A touch-sensing-enabled active matrix LCD display panel (or touchscreen) is an active matrix LCD panel having an additional function of detecting a location of a possible finger and/or pointing device touch made on the panel. A capacitive touchscreen comprises an insulator such as glass, coated with a transparent conductor. As the human body also conducts electricity, touching the surface of the screen results in a distortion of the screen's electrostatic field, measurable as a change in capacitance. Various technologies can then be used to detect the location of the touch point; for example, projected capacitive touch (PCT) technology.
PCT employs an X-Y (or column-row) grid of electrodes, comparable to the pixel grid in a LCD panel. A common voltage is applied on each of the column or row electrodes (the driving line), creating a uniform electrostatic field. When a conductor such as a human finger or a conductive stylus comes close to a point on the grid of electrodes, it distorts the local electrostatic field at that point, and this distortion is measurable as a change in capacitance. The capacitance change at every individual point on grid of electrodes can be measured to accurately determine the touch location by measuring the sensing signal voltage charge in the electrode of the other axis (the sensing line).
In conventional PCT type capacitive touchscreen panels, the sensing signal voltage charge is to be amplified by a voltage charge amplifier in the analog front-end circuit before it is usable by touch controller application. FIG. 1 shows a conventional voltage charge amplifier. The voltage amplification is then characterized by the following equation:
                              V          out                =                              V            ref                    -                                    R                              1                +                sRC                                      ⁢                          (                                                i                  cm                                +                                  i                  noise                                +                                  i                                      Δ                    ⁢                                                                                  ⁢                    c                    ⁢                                                                                  ⁢                    m                                                              )                                                          (        1        )            where iΔcm is the sensing signal current when touch is detected; icm and inoise are the common mode current and noise current respectively.
As can be seen in equation (1), the use of conventional voltage charge amplifier presents at least two problems:                1. The voltage headroom of the output voltage Vout is limited by the supply voltage to the operational amplifier and the size of the integrating capacitor C. In many cases, the supply voltage attributed by the common mode and noise current occupies more than 90% of the dynamic range of the operational amplifier. This creates a poor signal-to-noise ratio (SNR) as a relatively large voltage range is used to accommodate noise, leaving only a small fraction of the voltage range for the sensing signal current iΔcm.        2. The size of the integrating capacitor C is necessary to be large to attenuate the operational amplifier output voltage in order to accommodate the common mode and noise currents and the sensing signal current iΔcm. This leads to a large die size for the analog front-end circuits.        